<title>Simple annular resonators with toric and helical mirrors</title>

Abstract

Two simple annular resonators are described each consisting of two total reflecting annular mirrors with radially concave curvatures. The beam is guided to a coupling aperture in azimuthal direction by the azimuthal mirror shape.A resonator with tilted toric mirrors works like an azimuthal unstable resonator with two beams starting from a core oscillator of this resonator towards the coupling aperture. The function of this resonator is akin to conventional positive branch unstable strip resonators wrapped to an annular geometry.A resonator with helix-shaped mirrors emits one well defined beam. A numerical model of the resonators using a 3D-diffraction code and including effects of misalignment and saturable gain is presented. Numerical beam propagation techniques based on different approximations are compared. Due to the high azimuthal Fresnel number geometrical optics becomes applicable and turns out to be useful for a rough estimation of resonator design parameters.Experiments have been carried out using a diffusion cooled transverse rf-excited coaxial C02-lasergas discharge. Experimental data are compared with theoretical results concerning near field-, far fieldpatterns as well as misalignment sensitivity. Theoretical and experimental data correlate well.Both resonators yield more than 10 % efficiency. Using the tilted mirror resonator laser output powers of 2 kW have been obtained.